What are the techniques used to recover data?

Data Recovery process may vary from quite simple software recovery techniques to the chain of various sophisticated technological techniques involving processing disks with fine mechanic tooling in the clean room, reconstruction of destroyed elements of the drive - such as electronic circuit elements or pieces of original firmware.

So, basic recovery techniques can be divided into following groups:

• Software Recovery Techniques
  These techniques do not work on the physically (mechanically) damaged units. Therefore, can be only applied to operative (normally functioning, working correctly) device, any attempt using these techniques on the malfunctioning drive will only deteriorate the situation. But generally speaking, this is a very important and essential final phase of any complex data recovery process. Same recovery principles are implemented in some core OS utilities - supplied with operating system, such as disks Disk Utility for Mac computers or System restore in Windows 10. These kinds of computer programs can fix certain disk problems by using pure software tools - for example, multiple apps quit unexpectedly, or some file are missed or corrupted, and, sometimes, even when an external device quit working properly or your computer won't start up.
  Caution! None of the recovery technique is able to restore data from overwritten sectors. It happens when the physical location of your file on the disk (chain of LBA sectors) was written over by other data. Many programs write - silently and practically non-stop - a lot of logs, sessions, incoming email, and other information on your computer. To preserve device from any writings drive must be electrically disconnected from the power supply (unplugging the cord and removing the battery are first things to get on with).
• Clean room recovery techniques
  Mechanical repairs involve the disks, motor, or head replacements, fine tuning, balancing, etc.
• Microelectronic repairing techniques
  involve PCB rework and repair
• Firmware reconstruction techniques
  involve developing software and hardware tools to bypass damaged pieces of internal program code
• Direct raw scanning techniques
  involve reading digital information from inaccessible device for the following data processing